Increased soil moisture intensifies the impacts of forest-to-pasture conversion on methane emissions and methane-cycling communities in the Eastern Amazon.

Climatic changes are altering precipitation patterns in the Amazon and may influence soil methane (CH) fluxes due to the differential responses of methanogenic and methanotrophic microorganisms. However, it remains unclear if these climate feedbacks can amplify land-use-related impacts on the CH cycle. To better predict the responses of soil CH-cycling microorganisms and emissions under altered moisture levels in the Eastern Brazilian Amazon, we performed a 30-day microcosm experiment manipulating the moisture content (original moisture; 60%, 80%, and 100% of field capacity - FC) of forest and pasture soils. Gas samples were collected periodically for gas chromatography analysis, and methanogenic archaeal and methanotrophic bacterial communities were assessed using quantitative PCR and metagenomics. Positive and negative daily CH fluxes were observed for forest and pasture, indicating that these soils can act as both CH sources and sinks. Cumulative emissions and the abundance of methanogenesis-related genes and taxonomic groups were affected by land use, moisture, and their interaction. Pasture soils at 100% FC had the highest abundance of methanogens and CH emissions, 22 times higher than forest soils under the same treatment. Higher ratios of methanogens to methanotrophs were found in pasture than in forest soils, even at field capacity conditions. Land use and moisture were significant factors influencing the composition of methanogenic and methanotrophic communities. The diversity and evenness of methanogens did not change throughout the experiment. In contrast, methanotrophs exhibited the highest diversity and evenness in pasture soils at 100% FC. Taken together, our results suggest that increased moisture exacerbates soil CH emissions and microbial responses driven by land-use change in the Amazon. This is the first report on the microbial CH cycle in Amazonian upland soils that combined one-month gas measurements with advanced molecular methods.